1. Field of the Invention
The invention refers to a flexible joint for connection of two pipes, the flexible joint including a cup-shaped piece connected to one pipe, a tubular piece connected to the second pipe, coaxial with the cuplike piece and arranged partly inside it, and two flexible spherical rings arranged between the cuplike piece and the tubular piece.
2. Description of the Prior Art
In drilling oilwells at sea, an extension tube called a "riser" is placed between the blow out preventer of the wellhead on the sea bottom and the drilling installation on the surface as, for instance, a drilling platform or ship. Through this riser tube passes the drilling string and the drilling mud rises through it. Consequently, the riser must have a sufficiently large diameter for enabling the drill bit and the various devices employed in drilling to pass through, and a thickness capable of withstanding pressures which may reach several hundred bars. These imperatives lead to the employment of a tube of great rigidity. When drilling is conducted from a floating platform or a ship, the "riser" tube must be able to follow the movements without undergoing serious stresses. The vertical movements of the drilling platform are compensated by a suspension device of the "riser" tube which keeps the latter under near constant tension in order to avoid any risk of buckling. The horizontal movements of the floating support take along the tube which sweeps a conical volume the angle of which depends upon the accuracy of the positioning. Generally, the tube is equipped in its lower portion with a flexible joint which allows its oscillation and withstands the tension caused by the suspension device. The exploitation of movable platforms, drilling ships and oscillating platforms, has enabled it to be found that, although a device for suspension of the riser tube may be provided, compressive forces occur which are carried by the flexible joint and cause it to be put out of action prematurely.
Numerous patents have attempted to solve the multiple and often contradictory problems which are posed by the employment of flexible joints.
Thus, the French Patent No. 2 389 063 describes a flexible joint system which includes a system of distributing the difference between the outer and inner pressures, the inner pressure being approximately twice the outer. This joint comprises a cup-shaped piece connected to the lower pipe and a tubular piece connected to the upper pipe. This piece is arranged coaxially and partly in the cup-shaped piece where it is held by a double system of flexible spherical rings. The rings consist of an elastic body interposed between two endpieces, an inner one and an outer one, the spherical profiles of which are defined by arcs of circles having the same center or the centers of which are offset along the geometrical axis. The elastic bodies are formed of alternate layers of elastomer and an inextensible material, the thicknesses of which are constant or decreasing. The concave portions of the rings are arranged at one and the same side with respect to the center of rotation of the joint. One of the rings is fixed between a flange, which closes the opening in the cuplike piece, and the tubular piece and this ring works under compression in response to tensile forces. The second ring, fixed between the tubular piece and the bottom of the cuplike piece, likewise works in compression in response to tensile forces but the essential aim of it is to ensure sealing between the inside of the joint and the ambient medium. The volume lying between the seals and the inside of the cup is filled with an incompressible fluid which transmits the pressure between the rings and makes them act in series in order to distribute the difference between the internal pressure and the external ambient pressure. The outer ring likewise works as an emergency seal in case of failure of the inner sealing ring.
The French Patent No. 2 389 064 describes a similar flexible joint in which the two rings are arranged between the flange which closes the cuplike piece, and the end of the tubular piece. The tubular piece exhibits at the lower portion of it a spherical collar which cooperates with a spherical portion provided at the bottom of the cuplike piece in order to form a ball joint. This arrangement enables all of the forces of compression to be withstood, while there is a risk of tensile forces disengaging the parts of the ball joint. The flexible rings are arranged on opposite sides of the center of rotation and the centers of the arcs of circles which define the spherical profiles are located on opposite sides of the center of rotation on the geometrical axis.
These joints exhibit a certain number of disadvantages amongst which are:
the relative smallness of the angle of swing due either to the mechanical limitation of the travels by the opening in the flange or by the position of the center of rotation, or to lack of mobility due to the internal pressure;
the bulk: the outer diameter of the joint is large with respect to the useful diameter of passage;
limited working life due to the wear of the pieces in contact with the abrasive or corrosive products being conveyed;
the difficulty of erecting and dismantling such joints.
The French Patent No. 2 327 982 solves the problem of the swing by associating two joints of a type similar to that of the French Patent No. 2 389 063 but the device loses by this solution its simplicity and compactness.
The problem of bulk does not seem to have found any solution in spite of the interest which there is in reducing the areas subjected to pressure and the costs of manufacture.